Newbie - How to create solid from an extrusion and curve 90 deg to each other?

Hi guys and thanks for looking…

Newbie who knows what I need but haven’t yet been able to find a command to do it. Per the image I have an extruded shape and a half circle that are 90 degrees to each other.

I need to fill the space and have had some success with creating a surface from 4 curves but not good enough to resolve the issue. I’m continuing to try all I can find.

Any ideas gratefully accepted!



Update: The best I have managed thus far in a Sweep2 for the right side (perspective view) and a Patch for the left. There are a couple of odd bulges (arrowed) that I can’t seem to get rid of but are not show stoppers. Closer but if any of you experts have input, please let me know. Thanks!

Image 2:

Hi Goose,

I think Blend Surface will give you the results you want.

I approximated the look of your piece for this example, so I’m not positive it will give satisfactory results on your file, but if you haven’t tried BlendSrf, it’s pretty powerful.

Best regards,

surface between extrusions.3dm (213.7 KB)

Wow Doug, thanks for going to all the trouble to recreate my part.

I’ve spent quite some time with BLENDSRF now, thanks… it seems to do the trick and very powerful as you said.

One last question: My resulting blend seems to be ‘inside out’ (red outside, grey inside), is that an issue or is there a way to flip the normals?

Many thanks


Hi Simon,
The Direction command (Dir) gives you the option to flip the normals. When you Join all of the surfaces, Rhino figures out inside from outside and rationalizes all of the normals so they are all facing outward in a valid, closed polysurface.
Glad it’s working out for you!
Best regards,Doug

Perfect, thanks again.

Loving Rhino for it’s abilities to manage complexities, having a hell of a time with the basics… lot’s of learning to be done.


Hi V8Goose - for the sake of variety, here’s a different approach that tries to keep the character of the straight/extruded shapes intact as they sweep around the end arc:

surface between extrusions_PG.3dm (140.2 KB)

Sweep1. working up from blue to red, each sweep on the edge of the surface below. Use “Align with Surface” in the Style dropdown.


Wow, thanks Pascal…

This is how I envisioned a command working and I’ve successfully applied it.

It machined very well.

Thank you!

This is how I would do this. Mostly sweep 1 rail. Then project curves onto the top surface. Sweep 2 rail the helper cap surface. Then trim it back with a circular curve. Then redo the top swept surface this time as sweep 2 rails. This method does away with problematic surfaces with singularities (stacked cv’s).

On the other hand, singularities are only problematic if they are problematic - I don’t see any problem in this case, do you?


I have no idea in this case. Small surface ripples and surface continuity breaks around the singularity point. Combined with an increased risk of offset and shell failures. Has taught me that they’re to be avoided. I’ve figured out enough approaches. That it takes very little time to model around them. Dealing with them in fillets can be more time consuming. Even then it just requires advanced planning of the topology.

Stratosfear, can you show us your isocurves and explain more how you do not get a singularity in your green surface?

How does your Zebra look across that D cap? I have a feeling it would look a little funny where the singularities would have been in the D surface if you just did a blend. Are you saying trimmed surfaces are ‘better’ that untrimmed ones as far as reliability goes? I still haven’t been able to duplicate what you did. I get lost at projecting curves onto top surface. Which curves, which surface?

Trimmed surfaces are always better than untrimmed ones with singularity points. I did make some changes to existing geometry. Tangent matching the surfaces adjacent to the top cap.

This shows the problem with singularity surfaces. There’s a noticeable ripple and break in tangency. The greater the number of stacked cv’s the worse it becomes. The tangency break will often result in shell and thickening failures.

Hi Jason,

It should be noted that zebra analysis uses a mesh to visualise the surface normal continuities. Such a singularity results in a dense mesh around it, often with high aspect ratios of the mesh faces. The resulting artifacts in the rendering are not an accurate depiction of the surface normal continuities perse.


You are right depending on the mesh setting this can appear much worse than it really is. However the real issue comes when shelling and thickening. Where the the offset of the singularity surface will self intersect and create multiple naked edges.

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I am sorry but I am still a bit lost. My goal would be to turn the curves in this file into a closed polysurface with all surface edges joined tangentally and without singularities. I was hoping your methods would help me. I am basically a beginner and I still don’t see how you extended the top cap of the OP’s object (with square corners on the extension) such that the semicircle is on the surface of the extension.

test.3dm (39.5 KB)

Is there a way to to create my desired closed polysurface from the curves in my file?

Thanks for all the help so far!

This is the kind of singularity and non-tangental stuff I’ve have been able to achieve:

This is the topology approach I would take for this kind of form. I just figured out how to do this about a year ago. I speed modeled it and turned out water tight without the need for additional surface knots. I must be getting better. I had to add additional curves and clean up the existing ones to make everything bezier. In this case degree 3, 4 points.

I used the following commands: surface from edge curves, surface match, surface split at isocurve, surface shrink, surface blend. surface rebuild, curve duplicate edge, curve pullback, surface trim. It would take me forever to write a proper tutorial for this. Unfortunately I don’t have a capture card either. I hope this is helpful and gives you some insight.

You might give this a shot with t-splines. It should be easier and faster depending on your skill level. There are downsides to using t-splines. It just depends on what you are doing with the finished form.

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Thanks, I will make a serious attempt to follow your template, it does generally make sense to me.

I have 7 days left on my t-splines trial but I have lost faith in learning an easy way to build t-spline solids that pass through defined curve networks. But, I love t-splines for more loosely defined organic shapes!

Again, thanks for your help!